For many semiconductor applications there is a need to produce large single crystal films of high purity. Typical of these application are solar cells. In other applications even where small semiconductor devices are contemplated, economics dictate that the starting film size be as large as possible.
In the past thin semiconductor films were produced from bulk single crystals of the desired semiconductor by mechanically cutting the crystal into layers. Apart from the question of economics, such methods are technically undesirable because of the likelihood of producing significant amounts of contamination and mechanical defects into the layers. Thus, the semiconductor field has made use of various techniques for depositing thin semiconductor layers on substrates and removing such layers from the substrate by different techniques. This invention comprises a method and apparatus for depositing thin layers of semiconductor materials such as germanium and gallium arsenide on inexpensive disposable sodium chloride substrates. In contrast to the prior art, the invention provides for high rates of deposition without contamination of the semiconductor material by the substrate material.
There are two methods which have general utility in the production of thin semiconductor films. In the physical vapor deposition (PVD) process the starting material is the same material as the desired coating and deposition occurs without chemical reaction, for example by evaporation or sputtering. PVD deposition of semiconductors is described in U.S. Pat. Nos. 3,158,511, 3,186,880 and 4,255,208. All of these patents mention the use of sodium chloride as a substrate. The other major process is chemical vapor deposition (CVD). In the chemical vapor deposition (CVD) process the starting material is a chemical compound having as one of its elements the desired coating materials. The other elements in the compound are usually materials which are gaseous under the deposition conditions employed. This starting material, which is usually a gas is decomposed (usually thermally decomposed) and the desired coating material condenses on the (heated) substrate. U.S. Pat. Nos. 3,661,637, 3,993,533 and 4,171,235 all describe the CVD deposition of semiconductor materials. Inherent in the CVD process is the necessity to heat the substrate to a substantial temperature. The thermal energy provided in the substrate by heating serves two purposes, it serves to decompose the precursor gas and it also aids in the deposition process in the following fashion. The thermal energy increases the surface mobility of the semiconductor atoms after they strike the surface, thus increasing the likelihood that a semiconductor atom that strikes the substrate will come to rest in a "minimum potential well" so that epitaxial growth will occur. The necessity of heating the substrates reduces the processes flexibility in two respects. When certain otherwise suitable substrate materials are heated they sublime and redeposit on the substrate along with the semiconductor, thus contaminating the semiconductor. The necessity to heat the substrate to a substantial temperature also imposes the requirement that the substrate and semiconductor material have similar coefficients of thermal expansion. If the substrate and semiconductors have significantly different coefficients of thermal expansion, mechanical failure of the semiconductor film may occur when cooling to room temperature. In an apparent effort to overcome this thermal mechanical problem, certain patents suggest the use of a three layer technique in which a semiconductor substrate is coated with a thin layer of salt for which it has a sufficiently different coefficient thermal expansion and the semiconductor film is deposited on the intermediate layer. This is described in U.S. Pat. No. 4,116,751. Using this approach, the initial semiconductor substrate apparently provides substantial support and restraint so that mechanical failure of the thin film is not a problem.